A nitrile group-containing highly saturated copolymer rubber such as a hydrogenated acrylonitrile-butadiene copolymer rubber is superior in heat resistance, oil resistance, ozone resistance, etc. compared with a general nitrile group-containing copolymer rubber high in a ratio of carbon-carbon unsaturated bonds in its main chain structures such as an acrylonitrile-butadiene copolymer rubber.
As the composition of such a nitrile-group-containing highly saturated copolymer rubber, for example, Patent Document 1 proposes a cross-linkable rubber composition containing a highly saturated nitrile rubber which has α,β-ethylenically unsaturated dicarboxylic acid mono ester monomer units, a polyamine cross-linking agent, and a basic cross-linking accelerator. While a rubber cross-linked product which is improved in heat resistance and compression set is obtained by this composition, further improvement of the cold resistance has been desired.